Internet of Things, IoT, is expected to increase the number of connected devices significantly. A vast majority of these devices will likely operate in unlicensed bands, in particular in the 2.4 GHz ISM band. At the same time, there is also increased demand for using the unlicensed bands also for services that traditionally have been supported in licensed bands. As an example of the latter, third generation partnership project, 3GPP, that traditionally develop specifications only for licensed bands have now also developed versions of Long Term Evolution, LTE, which will operate in the 5 GHz unlicensed band.
Technologies that are expected to dominate for IoT services are Bluetooth Wireless Technology, in particular Bluetooth Low Energy, BLE, and future versions of IEEE 802.11 like 802.11ax. With respect to IEEE 802.11, there are currently efforts to standardize a Long Range Low Power, LRLP, mode which at least to some extent builds upon the above mentioned 802.11ax.
IoT applications are foreseen to most often have rather different requirement and features compared to applications like e.g. file download and video streaming. Specifically, IoT applications would typically only require low data rate and the amount of data transmitted in a single packet may often only be a few bytes. In addition, the transmissions to and from many devices will occur very seldom, e.g. once an hour or even less often. The number of IoT devices is expected to be huge, which means that although the amount of data to each one of the devices may be small, the aggregated IoT data may still be substantial. Many use cases for IoT applications can be found in an ordinary house, and may be related to various sensors, actuators, etc. The requirements for coverage are therefore substantially less than what usually can be achieved by e.g. a cellular system. On the other hand, the coverage which can be obtained by e.g. Bluetooth or 802.11b/g/n/ac may not suffice. This may be in particular true if one of the devices is outdoors whereas the other device is indoors so that an exterior with rather high penetration loss is in between the devices.
Due to this short-coming of current versions of Bluetooth wireless technology and IEEE 802.11, both these standardization organizations are working on new versions what would significantly increase the coverage.
The straight-forward approach to increase the range of a communication link is to reduce the bit rate that is used. Reducing the bit rate by necessity means that it will take longer to transmit a packet of a certain size. As a side effect of this, the channel will be occupied for a longer time. Now, with a large number of devices sharing the same channel, the channel may be congested if this sharing is not done in an effective way. The need for long packets and the increased number of users will make this congestion even more pronounced.
Moreover, the amount of non-IoT data, e.g. data down-load and video streaming, sent over the same channel may also increase. This implies that to obtain good performance for both IoT applications and non-IoT applications, some coordination should preferably take place.
An obvious, and probably the simplest, way to do such coordination is by time sharing between the systems.
However, as the data rate for the IoT system is very low for the individual links, it may likely be hard to obtain good spectrum efficiency in this way. Instead it would be preferable if the two systems, i.e., both the IoT system and the non-IoT system could operate concurrently. One means to achieve this could be if the non-IoT system would be based on Orthogonal Frequency-Division Multiplexing, OFDM. Concurrent operation could then be achieved by assigning one or more sub-carriers to the IoT system and the remaining ones to the non-IoT system. The amount of sub-carriers allocated to the IoT system could in this way be rather flexible.
The approach of using OFDM is conceptually simple and is also the approach suggested for the LRLP mode currently discussed within IEEE 802.11.
On the other hand, variants of Frequency Shift Keying, FSK, modulation are used in e.g. Bluetooth Wireless Technology. Bluetooth employs Gaussian Frequency Shift Keying, GFSK. GFSK is a constant envelope modulation which allows for extremely cost efficient implementations. At the receiver side, one may use a simple limiting receiver, i.e. the Analog-to-Digital Converter, ADC, may be replaced by a simple comparator and there will essentially be no need for Automatic Gain Control, AGC, in the receiver, further simplifying the implementation and reducing the cost. Even more significant is the gain at the transmitter side. Due to that GFSK is constant envelope, there is no need to back-off the power amplifier, PA, and there are no linearity requirements the PA, so that significantly higher power efficiency can be obtained. OFDM is known to suffer severely from a high Peak-to-Average-Ratio, PAR, which means that less efficient transmission. Since an IoT device, such a sensor, may be powered by a coin battery, power efficiency is one of the key features. Constant envelope OFDM has been introduced as a means to eliminate the high PAR. It is a non-linear modulation that involves modification of conventional OFDM by introducing a phase modulator at the transmitter, and additional FFT blocks, as well as rather advanced receiver algorithms at the receiver side.
Today there is no single standard that effectively supports both high-data rate application and really low cost IoT applications, like sensors. The main standard for the former is IEEE 802.11, e.g. 802.11n and 802.11ac, whereas the main standard for the latter is Bluetooth Low Energy, BLE. There is currently work ongoing to further improve the high rate support of 802.11, though the new 802.11ax amendment and work has also been started to enhance the support for IoT through the work on an amendment to support transmission from IoT devices. It has been identified that such an amendment should preferably be made such that IoT support in the Access Point, AP, can be added at essentially no cost by reusing selected parts of the key features of the physical layer from 802.11ax. Although this clearly is an attractive property, it does not address the even more important question namely how to build extremely low cost and low power devices. As 802.11 technologies, in particular 802.11ax, has some non-desirable properties for low cost and especially low power, it does not seem feasible for the low cost device.